1. Field of the Invention
The present invention relates generally to surveillance methods and apparatus for place sceneries or circumstances, particularly to place scenery surveillance methods and apparatus which utilize video cameras.
2. Description of the Prior Art
Conventionally, video camera placements for surveillance by video monitors at remote locations are a widely used method for labour savings or as a means of place communication for the purposes of place scenery surveillance such as inside of stores or parking places or further for surveillance of work spots within factories or the like.
At present, various attempts are made to automate such place surveillance by such video cameras which generate alarm signals only when there are occurrences of abnormal nature at such places. In this case, since the video camera picks up the scenery of the place by the light reflection at the place and delivers its video signal, if the light radiation thereon is not constant, automatic serveillance of the place, in other words, accurate detection of abnormalities becomes difficult. When the place of scenery to be surveyed is indoors, it is possible to maintain a steady light irradiation condition on the scenery to a certain extent, but when there is sunlight beaming into such indoors, this condition may not be steady. Further, for place serveillance at outdoors, the brightness of the place scenery varies by the weather conditions, and needless to say, there is the brightness difference related to time and the automation of surveillance becomes an extremely difficult task.
Generally, many attempts have been made to stabilize such incoming brightness to the video camera by automatically adjusting the exposure degree of the optical lens on the video camera to large or small in order to respond with such brightness changes, which is good if the variation in brightness is of a degree that such automatic exposure control can follow, but in practical cases such as the sunlight, the variation is so large that complete response adjustment cannot be expected. Further, even in the case of artificial lighting, the same can be said when the degree of changes are large. In other words, it was practically impossible to automatically conduct steady surveillances of abnormal changes of scenery, such as automobile movements at parking lots or people passing in or out of a place by such surveillance systems of the prior art.